In the art of magnetic recording, a patent 3,053,941 issued Sept. 11, 1962, disclosing a magnetostrictive transducer for recording and reproducing information on magnetic tape. The head uses a tube of magnetostrictive material enveloped by a "winding" of conductive material. The structure is placed under stress and longitudinal acoustic waves propagate along the tube causing the stress to change and creating a moving ring having magnetizable properties. A longitudinal non-magnetic gap adjacent to the magnetic tape is used for recording and reading the magnetic patterns onto and off the tape.
The trend in recent years has been towards higher frequency response and shorter gaps which require much smaller gap depths which, in the case of patent 3,053,941, relates to the tube thickness and, as a result, the idea set forth in this patent has become impractical because tubes having a wall thickness of 1 mil. or less would be required and these are mechanically unacceptable. Magnetostrictive materials can be deposited on non-magnetic substrates for mechanical rigidity, but the interaction between the speed of propagation in the substrate and the magnetic material is likely to cause perturbations and the materials are mechanically soft so that they will not be particularly wear-resistant making the head life very short.
The above-referred to patent 3,053,941 discloses a longitudinal wave which is propagated down the tube. There are two other types of acoustic waves which are well known and have been studied for use in several applications. In addition to the longitudinal wave there is a "shear wave" and a "Rayleigh wave". All three of these waves can, and in fact normally do, exist in a solid body that is being excited with acoustic energy. The Rayleigh wave travels slower than either of the two others and exists only near the surface of the material. The energy is concentrated in a relatively small region (approximately a wavelength in depth) and is assessable from the surface.
Surface acoustic wave (SAW) devices have been fabricated in the past as can be seen, for example, in an article entitled "Materials for elastic surface wave application" by J. D. Klerk on page 35 of the 1970 Ultrasonic Symposium Proceedings where surface of piezoelectric material is pulsed with an electrical signal to cause a surface wave to propagate down the length of the material. Such devices have been used as filters and for light scattering among other things.
Thin film magnetic heads that make use of nickel iron alloys are also well known and are in common use today as magnetic disc heads. Some nickel iron alloys are also highly magnetostrictive. Other alloys, as well as pure magnetic materials such as the metals nickel and iron, are also magnetostrictive. Although the description herein describes the use of nickel iron alloy as the magnetizable material, it is not intended that the invention be limited to this alloy.
Magnetostriction is commonly regarded as the change in stress in a material as a result of the presence of a magnetic field. The converse is also true. The magnetic properties of a material are changed as a result of induced stress in the material. It is this later phenomenon which is the more pertinent to the present invention.